This invention relates to vacuum devices as used for semiconductor equipment, such as thin film producing or etching units, and also can be used in other electronic, optical and experimental units. A metallic gasket is provided for use in a joint that connects parts of a vacuum vessel with a member, such as a viewing port, or for connecting such parts with each other, and achieving a vacuum state. The invention also relates to a method for producing the same.
In general, a gasket for a joint in a vacuum device comprises an elastomer, such as rubber, or a metal. A gasket made of elastomer may be feasible due to improvements in the quality of rubber for withstanding vacuum. However, a gasket made of metal is also used in many cases, such as where there are issues with airtightness at ultra high vacuum, high temperature conditions, or problems from gas that may be evolved from a gasket made of elastomer.
A metallic gasket can be generally shaped as a plate with the shape of a rectangle in cross section. As shown in FIG. 7(a) , a gasket in the shape of a plate 11 is held between a pair of joint parts 21 and 22 forming a vacuum barrier, each having a flange 2a and an annular knife-edge 2b. Bolts 3 are inserted into a number of holes 2c around flange 2a, and tightened with nuts 5, so that the knife-edges 2b bite into both sides 1a, 1a of the plate-shaped gasket 11. The joint parts 21 and 22 are joined to each other by the gasket, maintaining the airtight barrier.
Alternatively, as shown in FIG. 7(b), a gasket 12 having a cross section in the shape of a rectangle is held between a pair of vacuum joint parts 21 and 22, each having a flange 2a, a tapered face 2d broadening the opening diametrically outward, and a vertical face 2e at the outside diameter end. As above, the joint parts are fastened with bolts 3 and nuts 5, such that a tapered face 2d abuts on a corner portion 1c against gasket 12. The corner portion 1c is crushed, holding the gasket 12 between the tapered face 2d and the vertical face 2e. The gasket thus connects joint parts 21 and 22, holding airtightness.
Airtightness is maintained from squeezing deformation against both opposite faces of the gasket by knife-edges 2b, applied against the plate shaped metallic gasket 11, and by squeezing deformation at the corner portion 1c of the gasket from the tapered face 2d of gasket 12. Generally, the metallic material of the gasket is copper, and aluminum may be also used.
A vacuum joint 2 that can be baked, having a knife-edge 2b or tapered face 2d (generally called a xe2x80x9cCon Flat flangexe2x80x9d where xe2x80x9cCon Flatxe2x80x9d is a trademark) can be used in combination with a gasket made of oxygen -free copper. This material is advantageous in that oxygen-free copper deforms with pressure to an extent that can be matched to the strength and pressing force applied at flange 2a, owing to the number of the bolts 3, with good results.
A desirable gasket in modern applications should endure heating of 200 to 450xc2x0, which is a generally applicable range of baking temperatures in ultra high vacuum equipment. The gasket should be corrosion resistant in a reducing atmosphere including a halogen or the like, as appropriate in some thin film producing processes. Furthermore, the desired gasket should endure use in an even more corrosive atmosphere, wherein process gas such as NFn and free ions exist, for example as typical of in-situ cleaning by an etching processing unit during semiconductor production. In such a case, nickel, its alloy or stainless steel, having high corrosion resistance and high heat resistance, is preferred material.
Experiments were made by the present inventors, using a gasket made of nickel, nickel alloy or stainless steel on the above-mentioned vacuum joints. Compared to a conventional gasket made of oxygen free copper, it was found to be difficult to maintain sufficient airtightness in a gasket made of nickel or the like, having an annular plate shape and a rectangular shape in section, and having a constant thickness. Moreover, it was found to be difficult to get sufficient airtightness even in an experiment with a nickel gasket having a surface ground to a mirror finish or one with a nickel base, coated with gold or silver.
FIG. 8(a) shows a magnified area of a gasket surface 1a and an abutting face 2j of a joint member 2, the joint member being shown schematically. As shown in FIG. 8(b), in case of the gasket made of oxygen free copper 1cu, the abutting face 2j of the joint member and the gasket surface 1a have been brought against one another in such a manner that the abutting face 2j of the joint is pressed against the gasket surface 1a. The respective high portions of the gasket (convex portions) a, b, c, d . . . independently bear against the abutting face 2j and are squeezed and deformed, so that the lower portions (concave portions) m, n, o, p . . . become confined below the abutment surface. By comparison as shown in FIG. 8(c), in case where the gasket is made of material that is less deformable by such pressure, such as nickel (and its alloy, stainless steel) 1ni, the high portions (convex portions) a, b, c, d are only slightly squeezed when supporting a comparable load. The whole gasket surface deforms, as opposed to the high points. Although pressure is applied across the whole gasket surface, the fine variations in the gasket surface substantially retain their uneven shape.
On the basis of these results, the inventors reasoned that if a flaw x, such as a scrape, a scratch or a dent, is formed on the surface 1a of a metallic gasket during production, transportation or storage (see FIG. 9(a)), the flaw x can be compensated (xxe2x80x2) if the gasket has sufficient malleability as in the case of a gasket made of oxygen free copper 1cu, shown in FIG. 9(b). Deformation of the gasket material by a knife-edge 2b (or a tapered face) in that case maintains airtightness. In the case of a gasket made of less malleable nickel (and its alloy, and stainless steel) 1ni, however, a flaw xxe2x80x3 remains and can result in communication between an inside and an outside of the vacuum device. Even if the pressure from bolts 3 is increased to a large pressing force, the result is to depress the surface including the flaw x, with deformation as shown in FIG. 9(c). The joint may fail to prevent communication. In the respective views of FIGS. 9(a)-9(c), the flaws x, xxe2x80x3 are exaggerated. In general, a flaw xxe2x80x3 of FIG. 9(c) is not on the plane of a cross section as shown, but is three-dimensionally formed, providing a path between the inside and outside of a vacuum device.
A metal having high corrosion resistance, such as nickel or stainless steel, has high hardness. Therefore, increasing the pressure on a metallic gasket material of corresponding hardness, by increasing the number of the bolts 3 so as to increase fastening force, may adversely affect the durability of the vacuum joint parts 21, 22, such as the knife-edge. As a result, the lifetime of the joint is substantially shortened. A thermal treatment, such as annealing, is preferably performed on metal having high corrosion resistance, such as nickel, so as to lower its hardness. Metallic gaskets are put together and/or are carried on a jig during such thermal treatment (e.g., baking). This can leave a flaw at the contact surfaces of the metallic gaskets when the gaskets (or gaskets and jigs) are pulled apart from one another after thermal treatment. Such a flaw has a bad influence on the sealing effectiveness of the metallic gasket.
An object of the present invention is to provide a metallic vacuum gasket and a method for producing it, wherein the gasket has a structure that does not form flaws at the surface that is to provide a sealing face, the gasket being otherwise as described above, thereby solving the production and installation problems mentioned.
The invention provides a metallic gasket for a vacuum device to be put between vacuum joints for sealing, the gasket being squeezed and deformed by pressure from an abutting portion of said joint. The gasket has a sealing face to be squeezed and deformed by being pressed by said abutting portion, at an area wherein the gasket is thin in a thickness direction compared to a land portion which is not positioned for contact with said abutting portion. The relatively thicker land portion protects the sealing face and prevents formation of a flaw on said sealing face.
According to this aspect, the sealing face is protected by the land portion even if two or more gaskets or a gasket and a jig become stuck together during production, for example during thermal treatment. The sealing face also is protected if the metallic gasket is abutted against other goods or rested on each other in a stack for carrying or storage. The sealing face is protected if a dent or a scratch is formed on a surface of the gasket owing to the intervention of chips. Accordingly, flaws are prevented from being formed, whether by flaking or by contact or by chips. Dents or scratches detracting from airtightness can be avoided.
The metallic gasket can comprise nickel, nickel alloy or stainless steel, with high corrosion resistance and durability suitable for use in a vacuum device under with a highly corrosive atmosphere. Although the malleability of ductility of such materials may be low, and sealing leakage therefore is of concern for the reasons discussed above, the incidence or dent or scratch damage to the sealing face is reduced. Sufficient airtightness is likely to be obtained.
At least the sealing face of the gasket can be plated with gold or silver. When plated with gold or silver and also being comprised of nickel, nickel alloy or stainless steel having high corrosion resistance, the gasket can have even higher corrosion resistance. Airtightness is improved due to the gold coating or silver coating and due to the shape characteristic concerning the protection of the sealing face by the land portion.
For use with a metallic gasket as in FIGS. 1(a) (b), (c) and FIG. 2, for instance, the metallic gasket can seal with a joint part for a vacuum device, the joint part comprising a knife-edge. The metallic gasket shown has a plate shape and on both opposite sides has a sealing face to be engaged by a knife-edge. The sealing face can be formed by a concave groove or a stepped portion, lower than the land portions on each of the opposite sides.
Accordingly, the sealing face on each of the opposite sides of the metallic gasket, which is lower than the land portion, is protected by the land portion from contact other than engagement of the sealing face by the knife edge. A reliably airtight seal can be obtained.
Referring to FIGS. 1(d) and 3, for instance, an abutting portion of one of the joint part that bears on the metallic gasket, and the metallic gasket, can comprise a tapered face. The metallic gasket can have a rectangular or almost rectangular cross section. The portion of rectangular cross section is abutted by a tapered face. The sealing face of the gasket can abut against the tapered face, and in cross section can include a corner portion on the inner diameter side that is obliquely cut off.
The tapered face of the gasket portion abutting on the tapered face of the joint part, namely the sealing face along the corner portion on the inner diameter side that is obliquely cut off, is below the part of the metallic gasket having a section in almost rectangular shape, and therefore is protected. Reliable airtightness is obtained.
Referring to FIG. 4, for instance, a least a portion of the metallic gasket at the sealing face can be formed with rotation cutting streaks having a fine pitch. The rotation cutting streaks can be pressed along the abutting portion on the annular joint side, such as by a knife-edge or a tapered face, the abutting portion squeezing and deforming the projecting portions of the streaks and improving airtight reliability.
The invention also concerns a method for producing a metallic gasket and vacuum seal. See FIG. 5, for instance. The metallic gasket is placed between joint parts of the vacuum device, for sealing with the gasket by being squeezed, deformed or pressed by an abutting portion of said joint parts.
A cutting process can be used to form the sealing face to be squeezed and deformed by pressure from the abutting portion. This process makes the sealing face thin in a thickness direction compared to a thicker land portion. The land portion is a portion of the gasket that is not to be abutted by the abutting portion for making the seal.
A thermal treatment process can be performed, the thermal treatment adjusting a hardness of the gasket in vacuum after the cutting process. The hardness of the metallic gasket can be adjusted to a proper level by heat treatment. Oxidation of the metallic gasket can be inhibited, and gas included in the gasket material can be removed, if the heat treatment is performed in vacuum. This improves the effectiveness of the metallic gasket. Inasmuch as the cutting process is performed before the heat treatment process, the hardness of the gasket can be made adjusted to the proper hardness by subsequent heat processing even if the surface of the gasket was hardened as a result of the by the cutting machining operation.
Even if two or more metallic gaskets are stuck together, or if a metallic gasket and a jig are stuck together by baking during the heat treatment process, the affected portion is the land portion. Baked portions may peel or be flaked off with a tool. Flaws formed by the appliance or by a tool when peeling off portions occur on the land portion. The sealing face is protected and kept without flaws. Whereas the sealing face is protected from flaws but nevertheless has been adjusted for hardness by the heat treatment, the resulting metallic gasket has high sealing efficiency and superior reliability.
More particularly, the method comprises producing the metallic gasket of nickel, and adjusting the hardness of the metallic gasket to a Vickers hardness 75 to 95 Hv by annealing in said heat treatment process. The nickel metallic gasket is softened by annealing in the heat treatment process so as to have Vickers hardness 75 to 95 Hv. As a result, the fastening force needed between the joint parts and the gasket for the vacuum device is reduced while obtaining a sufficient sealing function and corrosion resistance. Thus the process does not detract from the lifetime of the joint.
In another embodiment the metallic gasket is made of stainless steel. The hardness of said metallic gasket is adjusted to Vickers hardness 120 to 180 Hv by annealing in said heat treatment process. Stainless steel has high corrosion resistance and is relatively inexpensive. The gasket is softened by annealing in a heat treatment process so as to get a Vickers hardness 120 to 180 Hv. The result is a metallic gasket having corrosion resistance which is practical and inexpensive.